EP0310489B1 - Absorbent pad for packaging - Google Patents

Abstract

The invention relates to the packaging field and the extended duration packaging of food products, particularly by modifying their environing atmosphere by means of an absorbing structure containing hydroreactive products. The structure of the invention absorbs liquids, exsudates or condensates, and generates an active element by reaction of the agent with the liquid. It is characterized in that it is comprised of at least one matrix formed, by aeraulic process, of a substantially fibrous mixture of hydrophilic and hydrophobic materials wherein said hydroreactive agent is distributed.

Description

The present invention relates to the field of packaging, and is aimed at the long-term packaging of products, in particular food products, by modification of their surrounding atmosphere by means of an absorbent element containing hydroreactive products.

The techniques currently known for producing materials allowing the control of the atmosphere confined in a package call for either the use of absorbent bottoms to separate the exuded or condensed water from the conditioned product, or the use of active agents, such as desiccants or bacteriostats, in solid powder in a sachet.

The two methods are sometimes implemented jointly by mixing absorbent materials with active materials or by depositing the latter on an absorbent support by impregnation or coating.

Indeed, for the conservation of a product, food or not, in a packaging, the first condition is to separate the collected liquid. The easiest way is to create a double perforated bottom, as described in British patent 1131848. Other trays contain sheets of absorbent paper. US Patent 4,275,811 describes an absorbent material coated with a perforated impermeable sheet to separate the food product to be preserved from the absorbent wet material. In patent application JP 61/058658 or US patent 4615923 the absorbent bottom contains hydrogelling materials capable of retaining aqueous liquids. To better separate the liquid from the product, it is sometimes proposed to have a double interior perforated bottom as in patent application FR 2586653.

To control the atmosphere, different reagents are used depending on the desired effect. Thus the packages often contain sachets of silica gel or calcium chloride to dehydrate the atmosphere, that is to say to trap the steam water into liquid water. Often, the oxygen in the package should be absorbed. Patent GB 496935 describes an oxidizable metallic product or ferrous sulphate mixed or not with other substances, such as citric acid, to slow down the action of microorganisms.

Patent application EP 206343 presents a mixture of iron powder and electrolytic agent, in the form of paper. Other patents complete the range of products capable of absorbing oxygen such as sulfites (US 2,825,651), bisulfites (US 3,169,068), or metal oxides (US 3,361,531) resulting from the decarbonation of metallic carbonates, or else ascorbic acid or its salts (French patent 2,394,988). The absorbent agents mentioned are fibers, clays, bentonites, kaolins, active alumina, activated carbon. Patent application JP 80/29975 describes an impregnated material covered with an oxygen absorbing agent and a carbon dioxide generating agent. Sometimes it is useful to generate oxygen by the action of water, as in patent application JP 61/145270 where the powder is contained in a sachet. Patent EP 128795 describes a tray bottom containing, in an absorbent means, not only a material capable of giving off a gas under the effect of an aqueous liquid, but also components of the foodstuff allowing, by aerosol effect, '' inhibit its degradation. The absorbent material is a sheet or a tea bag.

To avoid the disintegration of the active agents by the liquid, the presence of an absorbent material is necessary even in the sachets containing these agents in the form of powder or granules. To be able to reuse the desiccant, US Pat. No. 4,615,823 describes a composition in which the deliquescent salts are kept in a bag by means of fibers, while the Japanese patents 56/143263 and 61/103523 describe the retention of the saline liquid in the sachet. thanks to gelling agents.

The active agent can be a bactericide included in a cellulose sponge as described in patent GB 863095.

Among the means intended to control the atmosphere of a packaging, the invention is aimed at those which become active under the action of a fluid either released by the product which one wishes to preserve, or resulting from the condensation of vapor.

The invention proposes to solve the problem which consists in prolonging the action of these means as long as possible. Analysis of the prior art has shown that this problem has not yet been addressed.

When the powders or granules are enclosed in a sachet, they react as soon as the liquid has passed through the wall of the latter. The production of gas or active element is therefore uncontrolled.

In the case where liquids are emitted gradually, during the entire shelf life of a food period for example, it is possible to prolong the action of these elements by placing an amount in excess relative to that of the liquid which must to be collected. But the products are relatively expensive and this solution is not economically attractive.

In addition, in a sachet, the powder tends to gather in lumps under the effect of humidity. A part thus loses its accessibility to the liquid and becomes ineffective.

Finally, the manufacture and filling of the sachets are costly operations. This type of support is not economically profitable in view of its use in food packaging trays, for example, of which it should represent only a small part of the value.

If we take into account the technique of coating on a support, we come up against the problem of the very limited quantity of reagents that it is possible to fix, by this means, per unit of surface. It is also not possible to control the effect of the product over time. It is also known that certain water-reactants cannot be applied as an aqueous solvent by impregnation or coating without being altered.

In accordance with the invention, these drawbacks are remedied by producing an absorbent structure containing at least one water-reactive agent, intended to be placed inside a packaging, of foodstuffs in particular, to absorb liquids, exudates or condensates therefrom. in contact with which the agent reacts and releases at least one active element, characterized in that it consists at least of a mattress made up of an essentially fibrous mixture, formed by the aeraulic route, of hydrophilic materials and hydrophobic materials, inside which said hydroreactive agent is distributed.

• La répartition de l'agent, qui est sous forme de poudre ou de granulés, à l'intérieur d'un matelas de fibres évite tout risque d'agglomération en mottes. Le produit ne perd ainsi pas de son efficacité. Cette structure offre la possibilité d'incorporer une quantité relativement importante de poudre ou granulés.
• Il est possible, par un choix convenable des quantités de matières hydrophiles et hydrophobes, d'ajuster la vitesse de mouillage de la structure, et ainsi de contrôler la durée de réaction. Dans le cas de conservation des denrées alimentaires, la date limite de conservation (DLC) pourra être reculée de façon significative et fiable car on pourra garantir une action de longue durée sur l'atmosphère confinée dans l'emballage.
• Le fait que le matelas soit formé par voie aéraulique, c'est-à-dire que les matières fibreuses soient transportées dans un flux d'air, puis déposées sur un tapis, avec prémélange éventuel des poudres, permet des cadences de fabrication très élevées et rend de la sorte son coût très faible. Par ailleurs les matières formant le support de la poudre peuvent être choisies très bon marché.
• Le procédé par voie aéraulique permet de ne pas employer d'eau lors de la formation ou si besoin est, pour le renforcement du matelas, ce qui évite une éventuelle dégradation même partielle de l'hydroréactif par l'eau.

Such a structure has the following advantages:

• The distribution of the agent, which is in the form of powder or granules, inside a fiber mat avoids any risk of clumping. The product does not lose its effectiveness. This structure offers the possibility of incorporating a relatively large quantity of powder or granules.
• It is possible, by a suitable choice of the quantities of hydrophilic and hydrophobic materials, to adjust the rate of wetting of the structure, and thus to control the reaction time. In the case of food preservation, the expiry date (DLC) can be extended significantly and reliably because we can guarantee a long-lasting action on the atmosphere confined in the packaging.
• The fact that the mattress is formed by air, that is to say that the fibrous materials are transported in an air flow, then deposited on a carpet, with possible premixing of the powders, allows very high production rates. and thus makes its cost very low. Furthermore, the materials forming the support for the powder can be chosen very inexpensively.
• The aeraulic method makes it possible not to use water during the formation or if necessary, for the reinforcement of the mattress, which avoids a possible degradation even partial of the hydroreactive by water.

This structure is preferably covered on each of its faces with a veil or a film, permeable or impermeable as required, so as to control the penetration of liquids inside the mattress.

According to a preferred form of the invention, the absorbent complex can be reinforced according to known techniques using hot-melt fiber or powder binders. It is also possible to use aqueous latexes applied on the surface by spraying or in the form of a foam. When the complex is thus consolidated, it is possible to avoid having to coat it on one side of a veil or a film.

According to another advantageous form of the invention, the mattress containing the hydroreactive materials can be combined with at least one second absorbent mattress consisting for example of bonded fibers, whose wettability is different from the first mattress, and which will form a buffer element between the source of liquids and the structure containing the reagents, so as to increase the delay effect, and to control the distribution and the action of the liquid towards the other mattress.

The absorbent mattress containing the hydroreactive consists of at least one hydrophilic material and another clearly less hydrophilic, that is to say hydrophobic compared to the first. In addition, at least one of these components must be fibrous.

You can choose from the following mixes:

Cellulosic fibers include natural fibers, such as cotton or wood, made hydrophilic or hydrophobic depending on the nature of their treatment which determines the amount of residual hydrophobic resin, or regenerated fibers of rayon, viscose, etc. .

The term “treated synthetic fibers” includes synthetic fibers (PP, PE, PET, etc.) whose surface has been considerably modified to be made more wettable by water (for example PE fibers sold under the brand PULPEX by HERCULES). .

Hydrophilic fibers include hydrophilic fibers by nature such as glass fibers or metaphosphates.

The hydrophilic powder can be mineral or organic. Mineral powders include clays, vermiculites, glass, etc. Organic powders are of natural origin (guar, alginate, modified celluloses or starches, thickeners, viscosants or hydrogelifiers ...) or synthetic powders (polyethylenoxides, polyvinyl alcohols, polycarboxylates ...). These powders are more or less hydrophilic _ like fibers _ because they can be slow to wet (hydrophobia) and / or capable of gelling water (hydrogelifying powder) to retain it (hydrophilicity) which has the effect of slowing down the diffusion of water in the mattress _ desired character _ by the introduction of hydrophobic materials, presently classified less "hydrophilic".

Finally, the hydrophilic component of the mixture can be one of the hydrophilic components mentioned above or else a mixture of two or more of these. For example, it is possible to add a hydrophilic powder, in particular a hydrogelifier, to cellulosic fibers.

The ratio between hydrophilic and hydrophobic materials must be chosen so as to obtain a wettability of the product compatible with the intended application, that is to say the nature and the quantity of liquids present in the packaging. We will most often try to obtain the lowest possible wettability without making the product completely hydrophobic, that is to say that it cannot absorb liquid.

Thus, the amount of hydrophobic material can vary from 5 to 50% of the mass of the absorbent structure, excluding the hydroreactive agent.

It is important to define the terms hydrophilic or hydrophobic for which a product is designated.

The hydrophilicity of a product depends on its surface energy. It is known that a liquid wets all the better a flat surface that the difference of the surface energies of the liquid and the surface of the solid is small. Thus, water with a surface energy of 72 mJ / m² wets polyester (43mJ / m²) or cellulose (36 mJ / m²) or polypropylene (29 mJ / m²) well. Water does not wet teflon or PTFE, the surface energy of which is very low (18 mJ / m²).

The first surfaces are considered to be hydrophilic while PP, PE or, even more so, PTFE are known to be hydrophobic, because their surface energies are much lower than those of water.

In addition, a RHODORSIL 6356 silicone binder from RP (25 mJ / m²) is more hydrophobic than a phenolic binder at 42 mJ / m² or an epoxy adhesive (44.5 mJ / m²) EPIKOTE 828.

Unfortunately, the surface of the fibers is not flat and their roughness changes the contact angle of the liquid on this surface.

To characterize the wettability of a fiber, it is necessary to measure its contact angle with water for example according to the method of B. MILLER exposed in COLLOIDS AND SURFACES, 6, (1983), 49-61 consisting in measuring the angle of the liquid with the fiber coming into contact with the liquid and the angle of withdrawal when the fiber is removed from the liquid. For round fibers, MILLER has obtained the following values:

Thus this method makes it possible to evaluate the degree of wettability. When the angles are greater than 90 °, there is no wetting of the fiber by water. It is completely hydrophobic. On the contrary, a bleached kraft cellulose fiber would prove to be completely hydrophilic with very small contact and withdrawal angles. It follows from the above that the products designated by hydrophilic and hydrophobic (in the present invention) are products whose contact angles with water are significantly different. The difference in angles must be greater than 20 degrees, knowing that the so-called hydrophilic fibers will have the smallest contact and withdrawal angles, and less than 90 °, to merit the hydrophilic designation.

The measurement of the contact angle is decisive for judging the hydrophobicity or hydrophilicity of a material, fiber in particular, because a hydrophobic product can be treated superficially to make it hydrophilic and vice versa.

The hydroreactive agents, or hydroreactive materials, contained in the absorbent complex are, by definition, sensitive to water, liquid or vapor.

This reactivity in the physico-chemical sense results, for example, in the hydration of the material, its dissolution or its decomposition in the presence of aqueous liquid. The hydroreactive compound of the hydroreactive material is not necessarily the active agent of the composition.

Thus, water can decompose a hydrogenated compound into hydrogen which, in the presence of a catalyst, combines with oxygen to obtain an oxygen-free enclosure. (US patent 3419400). The hydroreactive compound can be the soluble capsule containing active agents.

The hydroreactive material is composed of one or more pure or mixed products, mineral or organic products at least partially soluble or capable of combining with water or of reacting or composing. These are, for example, mineral salts (NaCl, CaCL₂), organic salts (gluconates, etc.), bases (Ca (OH) Na, NaOH, etc.) or acids (citric, ascorbic acids), soluble materials ( sugar), oxides (CaO, MnO), peroxides (percarbonate 2Na₂ CO₃, 3H₂O₂), hydrides, metals encapsulated or not (Na, Fe ...), carboxymethylcelluloses ...

When the water-reactive material consists of two or more components, it is possible to distribute each of the components in a separate fibrous mattress or a layer distinct from the mattress which is thus multilayer.

The hydroreactive material can already be "stabilized", that is to say treated to be less reactive with atmospheric water, which also has the secondary effect of tempering and delaying the reactivity of the material with water.

The water-reactive material can be in the form of powder, granules, flakes, laminates, vermicelli or even fibers. By way of example, the behavior of three products according to the invention is described compared to that of two articles according to the state of the art, by means of three tests.

A first test makes it possible to evaluate the total absorption and the instantaneous absorption of the product. Obviously, in order to delay the reaction, a low wettability is sought. However, this remains sufficient so that, in practice, the liquid can be absorbed as it is emitted.

For this test, place a 6 × 10 cm sample on a horizontal plane. It is flooded with a certain amount of water. Then we tilt the support plane to 30 degrees while recovering the excess liquid flowing. By difference, the total absorption of the sample is calculated.

Then, place another sample identical to the first on the plane inclined at 30 degrees. Using a burette, instantly pour 50% more water than the product can absorb at saturation, and collect the excess. By difference we calculate the instantaneous absorption which can be expressed as a percentage of the total absorption and called wettability.

To assess the total reactivity of the product, we proceed according to the following second test. The sample product is placed in an Erlen-Meyer bottle which is closed by a stopper pierced with two holes. The first is surmounted by a bulb through which water is introduced in an amount 50% greater than the total absorption of the sample. The second is connected to a water pressure gauge on which the pressure due to gas evolution can be followed. The maximum pressure in water column (CE) and the total reaction time are measured, then the time it takes for the water column to drop to half the maximum value reached is counted; the reaction time is thus defined.

The purpose of a third test is to simulate the wetting of the product placed in the bottom of a container. The container is never strictly horizontal, so that the liquid generally flows on one of the edges of the product. The absorption is carried out by capillarity according to the law of Lucas, it is practically independent of the angle of inclination. The Klemm test makes it possible to follow the rise of the liquid as a function of time on a sample suspended on a balance by measuring the wet height and the quantity of water absorbed.

The total absorption is determined at the end of the experiment by immersion of the sample and then vertical drainage.

We can thus express the absorption at each instant as a percentage of the total absorption by immersion-draining.

EXAMPLE 1

A hydroreactive mixture of powders is prepared from three parts of an equimolar mixture of percarbonate and an organic diacid, and a part of neutral salt serving as an excipient, such as sodium chloride. The percarbonate used is stabilized. In an aqueous medium, it releases oxygen (13%) and carbon dioxide under the action of acid.

6 × 10 cm sachets are made with a nonwoven sold under the brand SERICEL, consisting of a plastic grid laminated with two sheets of cellulose wadding.

For the first test, 0.72 g is placed in a first sachet and 2 g in a second. It can be seen that in both cases the total absorption is 5.7 g and the instantaneous absorption 88%.

For the 2nd test, 2 g are again placed in a sachet. It is observed that the pressure rises rapidly and reaches its maximum of 33 cm of water column in a reaction time of 2 minutes. We can conclude that the gas evolution is instantly triggered by the presence of water, without delay effect. The pressure then decreases by dissolution of the gases. The reaction time is counted at 60 minutes.

By comparison, 2 g of hydroreactive powder hydrolyzed with 120 ml of water cause a pressure increase of 38 cm C.E. in a reaction time of 5 minutes. The reaction time of the powder alone is approximately 35 minutes. This example, as well as example 2, shows that the reaction time remains unchanged and that the reaction time is less than doubled when the powder is accompanied by absorbent materials depending on the state of the art.

EXAMPLE 2

The same hydroreactive mixture, in the same quantity as in Example 1, is introduced into a sachet made by assembling a sheet of cellulose fibers bound by spraying with latex, of grammage 60 g / m² _ product sold under the brand HOMECEL by the Company KAYSERSBERG _ and a sheet of SERICEL grammage 60 g / m².

The 6 × 10 cm sachet, 3.5 g, containing 2 g of water-reactive mixture _ structure in accordance with the teaching of the state of the art _ has an absorption of 9.7 g and a wettability of 88%. (without powder the wettability is 95%).

The sachet wets quickly, which has the effect of promoting the hydrolysis of the reagent and goes against the objective. In fact, according to the second test, it is found that the reaction time is shorter (3 min) for a maximum pressure of 28 cm C.E. The reaction time of 45 min is also shorter than in Example 1.

The third test confirms that this sachet is more wettable than the previous one, and the effectiveness of the water-reactive mixture is lower. The total absorption here is 7.5 g with a sample of 2.5 g. The absorptions are respectively:

65% after one minute

84% after ten minutes

84% after one hour

The absorption of this product is, after one hour, greater than 75% of the capacity.

EXAMPLE 3

In accordance with the invention, an absorbent complex containing the same hydroreactive material is formed. For this, we defibe pulp of cellulosic fibers (product sold under the brand Rayfloc J by ITT) and synthetic polyethylene pulp (fibers sold under the brand SWP by MITSUI) in the proportion 95/5. In the air stream charged with these fibers, 120% of hydroreactive powder of the mixture described in Example 1 is added. The whole is aspirated on a tissue paper of 18 g / m², calendered and thermolied in an oven at temperature of 30-150 ° C to allow the fusion of hydrophobic polyethylene thermobonding fibers. The whole is laminated with hotmelt glue, a polyethylene film on one side and a nonwoven on the other side

We can avoid sticking a nonwoven by replacing the tissue paper with a nonwoven made of two-component polypropylene-polyethylene fibers (Danaklon ES brand from JACOB HOLM), which allows the nonwoven to be bonded to the absorbent mattress by melting materials. polyethylene of nonwoven and absorbent mattress. The nature of the nonwoven, the size treatment of the fibers which compose it, the heat resistance of the size determine the wettability of the finished product.

_ papier tissue

 18 g/m²

_ fibres de cellulose

200 g/m²

_ fibres synthétiques

 10 g/m²

_ poudre hydroréactive

250 g/m² (47%)

_ adhésifs

  6 g/m²

_ film polythylène

 28 g/m²

_ non tissé

 18 g/m²

The composition of the 530 g / m² product is:

_ tissue paper

18 g / m²

_ cellulose fibers

200 g / m²

_ synthetic fibers

10 g / m²

_ water-reactive powder

250 g / m² (47%)

_ adhesives

6 g / m²

_ polythylene film

28 g / m²

_ nonwoven

18 g / m²

A 6 × 10 cm, 3.2 g sample absorbs 16.5 g and its wettability is 82%. The instantaneous absorption of a large quantity of liquid is slightly delayed which allows a strong capillary absorption.

The reaction time of the product according to the invention is considerably increased, 330 minutes, for an unchanged maximum pressure of 30 cm CE. The reaction time is then 485 minutes for this 3.85 g sample, containing 1.85 g of water-reactive material.

The absorption measurements by capillarity show that the wetting of the product is reduced, which allows a gentle hydrolysis of the reagents, for an absorption by immersion of 12.6 g (3.3 g of product).

The absorptions according to the third test are:

51%: after 1 minute

64%: after 10 minutes

64%: after 60 minutes

The absorption after one hour is much less than 75% of the capacity.

EXAMPLE 4

The procedure is as in Example 3, but by introducing more polyethylene fibers, 15% instead of 5%, into the fibrous mixture, in order to further modulate the reactivity of the product.

_ papier tissue

 18 g/m²

_ fibres de cellulose

160 g/m²

_ fibres synthétiques

 24 g/m²

_ poudre hydroréactive

144 g/m² (36%)

_ adhésifs

  6 g/m²

_ film polythylène

 28 g/m²

_ non tissé

 18 g/m²

The composition of the 400 g / m² product is:

_ tissue paper

18 g / m²

_ cellulose fibers

160 g / m²

_ synthetic fibers

24 g / m²

_ water-reactive powder

144 g / m² (36%)

_ adhesives

6 g / m²

_ polythylene film

28 g / m²

_ nonwoven

18 g / m²

The absorption of a 6 × 10 cm sample, of 2.2 g, containing 0.8 g of water-reactive mixture is 10.6 g and its wettability is no more than 6%.

A 7 × 12 cm 3.8 g sample containing 1.4 g of water-reactive mixture degassed in 150 minutes to reach the maximum pressure of 36 cm EC. The effect sought by the invention is clearly achieved, namely to allow a gradual release of gas, spread over time. For a reaction time of 150 minutes, the reaction time is 300 minutes. These values are clearly higher than those obtained with products according to the state of the art.

The absorption by immersion is 8.1 g for a sample of 2.7 g. The absorptions by capillary rise are respectively:

40% after 1 minute

57% after 10 minutes

57% after 60 minutes

The absorption after one hour does not exceed 75% of the capacity. It is lower than that of the product containing only 5% of hydrophobic fibers.

EXAMPLE 5

The procedure is as in Example 4, but the nonwoven covering one side of the product is replaced by a HOMECEL dry paper, already described, of 60 g / m². It is an absorbent nonwoven whose wettability is higher than that of the lower layer comprising polyethylene fibers, SWP.

This example aims to show that one can realize in one or more operations multilayer complexes. Other means exist for depositing fibers or powders locally, in the form of blocks for example, within a fibrous mass.

We can conceive a complex where the active agent is released by the reaction of two products A and B, at least one of which is water-reactive, these products A and B being judiciously placed in different layers or zones of complex in order to increase the delay effect.

The absorption of a 6 × 10 cm 2.4 g sample containing 1 g of water-reactive mixture is 13.7 g and its wettability, on the HOMECEL side, is 80%. This face is very hydrophilic opposite the more hydrophobic face containing the water-reactive material.

A 7 × 12 cm sample, containing approximately 1.3 g of water-reactive materials, reaches the maximum pressure of 10 cm EC in the presence of water in a reaction time of 1 minute. The reaction time is more than 300 minutes. The pressure remains stable between 8 and 9 cm C.E.

This complex where the water is divided between a zone without reagent and a reactive zone makes it possible to limit the evolution of gas and to obtain a long reaction time.

The third test confirms that this complex is more wettable. The total absorption being 11.8 g (2.8 g sample).

The absorptions are respectively:

54% after 1 minute

63% after 10 minutes

66.5% after 60 minutes

The absorption after one hour does not exceed 75% of the capacity.

These examples show the advantages of the products according to the invention compared to known products. Indeed, the hydrolysis times of the hydroreactive material are here more than doubled which was the desired effect, thanks to the use of selected absorbent materials, judiciously assembled.

This effect is obtained, according to the method described, by the use of 5 to 15% of thermoplastic fibers such as SWP from MITSUI or PULPEX from HERCULES.

It may also be spun fibers, cut, crimped or not, of polyethylene from DANAKLON or two-component DANAKLON ES from JACOB HOLM or even MELTY from UNITIKA. These fibers are usefully fused to bind the fibrous mat. The use of hot-melt powders, for example of polyester with low melting point of EASTMAN, makes it possible to obtain more flexible products.

The hydrophilic material is composed of natural or regenerated fibers or foams, fibers or foam or absorbent nonwoven based on hydrophilic treated synthetic material. The same partially treated material may have the desired characteristics combining hydrophilicity and hydrophobicity.

EXAMPLE 6

According to this embodiment, a sheet of fibers is first formed by a dry route, that is to say by aeraulic route. The powder is then deposited on the surface which is made to penetrate inside the sheet by blowing air, suction through the thickness of the sheet, or finally a combination of the two means.

This sheet containing all or part of the powder may be covered with an additional sheet, also formed by air flow, not containing or containing the other part of the hydroreaction powder.

A sheet of 300 g / m2 of cellulose fibers was thus produced containing 20% of polyethylene fibers and incorporating a water-reactive powder of the same composition as in Example 1.

This tablecloth was thermobonded by passage in the oven. The rate of dissolution of the water-reactant was monitored by measuring the speed of dissolution of the water-reactive after pouring 120 cc of water onto the sample.

Thus, while the hydroreactive alone produces a gaseous evolution at a pressure of 16 cm CE in a reaction time of 2 minutes, the sheet reacts regularly and for a longer time. The pressure of the gas released at equilibrium is 15 cm CE for a reaction time of 12 minutes.

EXAMPLE 7

Three layers of cellulosic fibers containing synthetic SWP fibers are produced by dry process at the rate of 0%, 5% and 15%, and powdered percarbonate. The tablecloths formed are passed for six seconds to the oven at 150 ° C.

To evaluate the dissolution rate of this peroxidized salt, the dissolution time is measured according to a fourth test, that is to say the time taken by a sample, immersed in distilled water, to obtain the balance of the solution of salt dissolved in water, with stirring. It can be seen that the dissolution times of the peroxide are increased by the presence of the hydrophobic fibers of the products P5 and P15 as well as, as before, the hydration times of the percarbonate.

EXAMPLE 8

As in Example 7, mixtures containing powdered citric acid are produced in place of percarbonate.

In the same way, the dissolution rate of the acid hydroreactor is controlled according to the method of the 4th test, by immersing the sample in distilled water previously brought to pH 8 with sodium hydroxide.

Claims (11)

1. An absorbent structure containing at least one water-reactive agent designed to be placed within the packaging of food products in particular in order to at least partially absorb liquids, exudates or condensates, in contact with which the agent reacts and releases at least one active agent, characterised in that it consists of at least one pad formed by air blowing of an essentially fibrous mixture of hydrophilic and hydrophobic materials within which the said water-reactive agent is distributed.

2. An absorbent structure according to claim 1, characterised in that the hydrophilic materials are fibres which may be mixed with hydrophilic powders.

3. An absorbent structure according to claim 2, characterised in that the fibres are cellulose fibres such as fibres of wood, cotton, viscose.

4. An absorbent structure according to any one of the foregoing claims, characterised in that the hydrophobic material consists of hydrophobic fibres, a binder or a powder.

5. An absorbent structure according to any one of the foregoing claims, characterised in that the pad consists, apart from the water-reactive agent, of 5 to 60% by weight of hydrophobic materials.

6. An absorbent structure according to one of the foregoing claims, characterised in that the water-reactive agent is in the form of powder, granules, flakes, sheets, short rods, fibres.

7. An absorbent structure according to claim 6, characterised in that the water-reactive agent included in the pad is in a ratio lying between 0.1 and 20 times by weight that of the pad not including the water-reactive agent.

8. An absorbent structure according to any one of the foregoing claims, characterised in that the pad is covered on at least one side by a permeable sheet or by an impermeable film.

9. An absorbent structure according to any one of the foregoing claims, characterised in that the pad is stabilised by means of binders.

10. An absorbent structure according to any one of the foregoing claims, characterised in that it includes a second fibrous pad having greater wettability than the first pad.

11. An absorbent structure according to any one of the foregoing claims, the water-reactive agent consisting of at least two components, characterised in that it includes at least two layers or two fibrous pads, each containing one of the components of the water-reactive agent.